Small electric power tools generally are made at a small size and a lower cost and can be coupled with different application tools to form various types, such as emery wheel machines, drilling machines, electric wrenches and the like that can assist workers to carry out some work processes quickly, thus are widely adopted. Electric power tools get driving power from electricity to drive motor which further drives application tools to rotate. U.S. Pat. No. 4,667,119 entitled “Trigger switch” discloses a trigger and switch for electric power tools. The trigger includes a sleeve and a pillar housed in the sleeve. The switch includes a set of switch contacts and another set of auxiliary contacts that are located sequentially on a movement path of the pillar. The trigger is depressible to move the sleeve and pillar to butt sequentially the switch contacts and auxiliary contacts (depending on the pressure) to provide different functions. In short, this prior art triggers a plurality of switches according to different depressing pressures to activate various functions.
Other references of trigger structures for electric power tools can be found in U.S. Pat. Nos. 6,749,028 and 7,638,725.
All the aforesaid trigger structures of the electric power tools have a trigger and a switch mechanism that respectively adopt a contact structure. While they are provided respectively with a staged control structure, the staged control structure of the switch mechanism is quite complicated. To achieve more precise multi-staged control effect, the switch structure becomes even more complex that results in a higher cost. Moreover, although the staged control can change output power or functions according to the depressing pressure, the wrench generally can provide merely two or three preset rotational speeds. The power or rotational speed are different among the stages and cannot be determined by users when in use, hence are not desirable for precise fabrication or handcrafted processes. For instance, on electric power tools for fabrication purpose, such as grinding machines, polishing machines, cutting machines and the like, usability and efficiency are affected by many factors, notably: 1. material hardness, density and heat resistance of the fabricated work pieces; 2. fine and coarse degree of fabrication interfaces and material characteristics; and 3. fabrication speed and quality requirements. Moreover, the speed and torque at the instant contact of tools and fabricated work pieces, during regular fabrication period, and separation thereof are different and also different while the tools are worked on a flat surface and non-flat surface. Hence to accomplish optimal fabrication effect output power of the electric power tools must be adjusted accordingly, such as the grinding machines, polishing machines, cutting machines and the like. All the aforesaid conventional techniques provide adjustment stages that are too few in number and too large in differences between them, thus are not desirable for precise and delicate fabrication processes.